The present invention is in the field of robot arm devices and more particularly relates to a multiple link control arm assembly.
Robot arm devices, or control arms, in the prior art are generally in the form of multiple link assemblies, having actuators for controlling the relative motion of the respective links in order to control the position of a control point in a work space. For example, a one-arm manipulator, or two bar linkage control arm, might control the position of a control point in a plane. Such manipulators include a first link member pivotably coupled at one end about an axis passing through a reference point on a support member. At its other end, the first link member is pivotably coupled to one end of a second link member. The control point is positioned at the other end of the second link member. Typically, a first drive motor on the support arm is adapted to control the angular position of the first link member with respect to the support member. A second drive motor mounted on the first or second link member controls the angular position of the second link member with respect to the first link member. The two motors may be selectively actuated to control the position of the control point in a predetermined work space. Such control arms have found use in industrial environments, where the control point may be coupled to a tool (such as a welder, or a cutting element) or to a manipulator (such as a limp fabric controller in an automated sewing, or garment assembly system).
One such commercially available device is the IBM 7540 arm. That arm is adapted so that its control point may be rotated about a fixed axis and a variable axis, which axes are spaced apart by the first link member. The position of the control point is controlled by two variables: the "first" angle between the first link member and a reference axis, and a "second" angle between the first and second link members. A work space is defined for the control point by limiting the first angle to the values of 0-200 degrees and by limiting the second angle to the values of 0-160 degrees. However, the work space envelope established with these constraints provides a relatively eccentric grid pattern requiring substantially different first angle and second angle displacements to achieve uniform motion of the control point within the work space.
Conventional control arms typically use servomotors as a power source for controlling the position of the control point in the work space. Such servomotors are generally operated at relatively high mechanical output frequencies and provide high output power for arm control. Since the required rate of angular motion of the arm components is substantially less than the output mechanical frequencies of the servomotors, step-down gear trains are typically used in order to provide the necessary speed reduction (from servomotor-to-arm). However, the gear trains introduce backlash, friction and mechanical stiffness, all of which place severe performance limitations on such control arms.
In an alternative approach, high-torque, low speed direct drive motors are used for arm control. While such direct drive motors, often in the form of brushless DC motors driven by PWM servo amplifiers, can provide suitable drive capability in some respects, there are still substantial limitations to such configurations. Particularly, static loading on the arm can cause overheating of the motor. Further, in serial link configurations, where a first drive motor may be mounted on the support member, and a second drive motor is mounted on a moving member, the weight of the second motor becomes a load on the first motor (requiring that the successive motors be correspondingly powered to accommodate such loading). Moreover, in such configurations reaction torques of the second motor acts upon the first motor.
Accordingly, it is an object of the present invention to provide an improved control arm for positioning a control point.
It is another object to provide for improved force-speed characteristics at the control point of a control arm.